Self-exciting optical strain gage

ABSTRACT

A self-exciting optical strain sensor (20) includes dual parallel bridges (22,24) having parallel facing surfaces (32,34). Light energy (38) entering a Fabry-Perot cavity (36) formed by this surfaces (32,34) induces a periodic buildup and release of energy in the cavity (36) which is directly related to the natural resonant frequency of the bridges (22,24). Analysis of the intensity of light emitted (50) from the cavity (36) determines the bridge natural frequency.

This is a division of application Ser. No. 923,770, filed Aug. 3, 1992,now abandoned.

FIELD OF THE INVENTION

The present invention pertains to an optical sensor responsive to strainin a substrate.

BACKGROUND

Optical strain gages, and in particular an optical strain gage useful ina pressure transducer, are known in the art. U.S. Pat. No. 5,101,664,Optical Pressure Transducer, issued on Apr. 7, 1992 to Hockaday, et alshows a micromachine silicon pressure transducer which employs a singlevibrating bridge and pressure responsive diaphragm formed from a singlesilicone wafer. The initial wafer is micromachined by a combination ofetching and laser techniques so as to result in a single strand ofsilicon supported at each end by blocks or other supports also cut fromthe initial silicon wafer.

By forming the diaphragm, supports, and bridge from a single slab ofhomogeneous material, a pressure transducer according to Hockaday, et alis virtually unstrained by changes in temperature due to the uniformityof the thermal expansivity of the integral structure.

The degree of strain experienced by the bridge in the Hockadayconfiguration is determined by exciting the bridge structure by the useof a beam of light which is pulsed in intensity at a frequency equal tothe nominal natural frequency of the bridge plus any strain-inducedchange in frequency. Optical devices, discussed in more detail in theHockaday patent, monitor the vibration frequency of the bridge throughthe use of reflected light analyzed by an interferometer, and varies thepulse frequency of the driving light beam until the resident frequencyof the bridge has been determined.

The amount of strain in the bridge is a function of the modulus ofelasticity, geometry, and longitudinal stress present in the bridge,which in turn may be easily converted to strain at the surface of thediaphragm. The prior art device thus provides an effective, simpletransducer for making temperature independent strain measurements, butrequiring a relatively complicated measuring system able to interpretthe measured frequency of the bridge and vary the driving frequency ofthe pulsed beam in order to seek out the current resonant bridgefrequency.

SUMMARY OF THE INVENTION

The present invention provides a self exciting optical strain sensorwhich is highly resistent to thermally induced strain. The sensorcomprises a pair of parallel bridge elements supported above asubstantially planar substrate, the substrate, supports, and bridgeelements all having been cut from a single, uniformly doped siliconwafer.

The sensor according to the present invention is formed from siliconhaving a <110> crystal structure. In this way, the facing walls of theparallel bridges are formed with substantially parallel alignment asthese walls must lie in the same family of <111> crystal planes.

The sensor according to the present invention is driven by anunmodulated beam of light, transmitted through one of the bridges, whichenters a Fabry-Perot cavity defined by the parallel facing bridge walls.By forming the facing walls with a proper initial spacing, an opticalintensity buildup will occur within the cavity when supplied by theunmodulated light. This optical intensity buildup causes a transversethermal gradient within the parallel bridge elements, which in turncauses lateral displacements of the bridge elements. This lateraldisplacement "detunes" the cavity, thereby reducing the opticalintensity therewithin.

As cavity optical intensity diminishes, the bridges' thermal gradientsare reduced, and the bridges return to the original spacing, and arethus prepared to repeat this cycle. The bridges and the intensity of thelight within the cavity thus oscilate at a frequency which is responsiveto the natural frequency, and hence imposed stress, of the bridges.

As noted above, the sensor according to the present invention and itsintegral supports and substrate, are cut from a single uniformly dopedsilicon wafer, thus eliminating differential thermal expansion. Anotheradvantage of the sensor according to the present invention is thesimplicity of the measurement apparatus used to drive and monitor thesensor. Due to the self excitation feature of this sensor, the driver isan unmodulated light source, while the vibration measuring apparatusmerely detects the cyclic intensity variation in the light reflected ortransmitted from the cavity.

The sensor is thus well suited for inclusion in a fully optical pressuresensor, wherein the silicon substrate forms the pressure diaphragm. Sucha pressure sensor would be electrically passive, small in size, andcapable of operating over an extreme temperature range without reducedaccuracy or calibration problems. Other potential applications of thestrain sensor according to the present invention can include,acceleration monitors, temperature monitors, or other devices, each ofwhich would use the same driving and monitoring apparatus discussedabove.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a prior art single bridge sensor configuration.

FIG. 2 shows a dual parallel bridge structure according to the presentinvention.

FIG. 3 shows a simplified schematic representation of the Fabry-Perotcavity and the reflected/transmitted light paths.

DETAILED DESCRIPTION

Referring to the drawing figures, and in particular to FIG. 1 thereof, aprior art, single bridge strain gage 10 as shown and described in moredetail in the reference of Hockaday described above. The single bridgeis formed from a single silicon wafer which is cut by means of chemicaland laser etching techniques to result in the single bridge 12 extendingbetween first and second end supports 14, 16 and suspended above asubstrate 18. It is also noted in the background section of thisspecification, the substrate 18 supports 14, 16 and bridge 12 areessentially a monolithic structure cut from a uniformly dopedcrystallized silicon wafer. The material properties are thus identicalthroughout structure the structure of FIG. 1 eliminating the possibilityof any differential thermal expansion, and hence the imposition of anyresidual or thermal stress between any of the aforementioned elements.

FIG. 2 shows a dual parallel bridge configuration 20 according to thepresent invention. Parallel bridges 22, 24 extend between supportmembers 26, 28 which support the bridges 22, 24 above the integralsubstrate 30. The bridges 22, 24 and supports 26, 28 may be formed by avariety of methods, including the laser chemical etchant processdescribed in the Hockaday, et al reference, as well as by a variety ofother micromachining techniques.

The individual bridges 22, 24 possess a natural resonant frequency whichis dependant upon the geometry of the individual bridges, i.e.cross-sectional moment of inertia and length, material properties, i.e.modulus of elasticity, and the residual or imposed longitudinal stresson the bridge elements 22, 24, i.e. restoring force. As the geometry andmaterial of the bridges remains substantially constant, the imposedlongitudinal stress exerted by the supports 26, 28 as a result of thedeformation of the substrate 30 results in a variation of the naturalresonant frequency of the individual bridges 22, 24.

The instant natural resonant frequency of the bridges 22, 24 is sensedby means of the optical arrangement shown schematically in FIG. 3. FIG.3 is a transverse cross-section of the bridges 22, 24 at a pointintermediate the supports 26, 28. The parallel facing surfaces 32, 34 ofthe respective bridges 22, 24 form a Fabry-Perot cavity 36 shown in thefigure. Surfaces 32, 34 are spaced apart a distance based on thewavelength of lights used to drive the sensor, as well as themanufacturability of the structures. Thus, a spacing of approximately 10microns, or roughly in the range of 10-20 wavelengths is believed toprovide good performance.

The Fabry-Perot cavity 36 is energized by means of a beam of light 38supplied, for example, by a fiber optic cable 40 which is directedagainst an exterior face 42 of the bridge 22. The light beam 38,unmodulated in intensity, is partially reflected 44 and partiallytransmitted 46 by the bridge 22. The surface reflectivity of a straingage according to the present invention formed from a typical siliconwafer is on the order of 30%. Thus, 49% of the light energy of theencountering beam 38 enters the cavity 36 whereupon it is both partiallyreflected 48 and partially transmitted 50 by the second bridge element24.

In the absence of the existence of the Fabry-Perot cavity 36,approximately 24% of the light energy in beam 38 would exit the outwardfacing surface 52 of the second bridge element 24. The Fabry-Perotcavity 36, however, achieves an optical intensity buildup by means ofconstructively adding the reflecting and re-reflecting 54 light withinthe cavity 36. This intensity, and hence energy, buildup in the cavity36 results in the creation of a transverse thermal gradient in each ofthe bridge elements 22, 24. The gradient causes a lateral deformation ofthe elements 22, 24 changing the spacing 56 between the facing surfaces32, 34.

By changing the spacing 56 between the surfaces 32, 34, the cavity 36becomes detuned, which means that the cavity 36 becomes unable toconstructively add the multiple reflections 54 thereby allowing theoptical intensity within the cavity 36 to drop and the resulting thermalgradients to disappear.

The bridge elements 22, 24 thus return to their original spacing 56thereby permitting a succeeding buildup of optical intensity within thecavity 36 and the resulting cyclical transverse deformation andrestoration of the bridge elements 22, 24.

Bridge elements 22, 24 thus are induced to vibrate at or near theirinstant natural frequency merely by the imposition of an unmodulatedlight beam 38 thereby achieving the self excitation feature recitedabove. The frequency of the vibrating bridges 22, 24 is measured bymeans of an optical intensity sensor 58 positioned opposite the fiberoptic transmitter 40 for measuring the intensity of light transmittedfrom within the cavity 36. As will be appreciated by those skilled inthe art, the variation of optical intensity within the cavity 36,occurring at a frequency equal to that of the vibrating bridges 22, 24,also causes the intensity of the transmitted light 50 to increase anddecrease in concert with the frequency of vibration of the bridges 22,24.

Alternatively, the frequency of the variation and intensity of the lightreflected and emitted 44 from the outward face 42 of the bridge 22, mayalso be measured by a suitable sensor 60, and will likewise have afrequency of intensity variation identical to the vibration frequency ofthe bridges 22, 24.

As noted in the Hockaday, et al reference for the single bridgeconfiguration, it is advantageous to form the dual bridge arrangementaccording to the present invention from a single slab of <110> silicon.In this fashion, the faces 32, 34 of the bridges 22, 24 will remainabsolutely parallel due to the configuration of the crystallinestructure of the silicon. This unfailing parallel alignment insuresproper operation of the Fabry-Perot cavity 36.

Exemplary dimensions of a dual bridge configuration according to thepresent invention would propose a pair of dual bridges, having a gap of10 microns between the facing surfaces 32, 34, each bridge having alength of 1000 microns, a height of 40 microns, and a thickness of 2.5microns. The value of Youngs modulus in the relevant orientation of<110> silicon is 1.69E11 N/m², while the density of crystalline siliconis 2.33E3 kg/m³. Calculations show the resonant frequency of thisstructure to be 22 kHz in the unstressed or no strain applied state.

As noted above, the structure according to the present invention, beingcut or formed from a single monolithic piece of silicon is virtuallyunaffected by temperature due to the uniformity of the materialcoefficient of thermal expansion throughout the structural elements.There is, however, a slight variation in the value of Youngs moduluswhich has been found to be -62.5 ppm/C. As will be appreciated by thoseskilled in the art, this thermally induced change in the siliconmaterial properties causes a slight change in the natural frequency ofthe bridge elements 22, 24.

I claim:
 1. In an optical pressure transducer having an externalhousing, said housing sealingly attached to the periphery of a diaphragmand defining a first cavity in conjunction with one side of thediaphragm and further defining a second cavity in conjunction with theother side of the diaphragm,wherein the improvement comprises, saiddiaphragm being a single, uniformly doped silicon wafer, said waferincluding substantially similar first and second parallel bridges havingsubstantially equal natural frequencies of vibration, each bridgeincluding a reflective surface facing a corresponding parallelreflective surface on the other bridge, said parallel facing surfacesforming an optical cavity therebetween, an optical fiber having a lightemitting end disposed adjacent one bridge for directing a beam ofunmodulated light energy through the one bridge and into said cavity,wherein the light is reflected between the first bridge surface and thesecond bridge surface, resulting in a buildup of the intensity of lightenergy within said cavity, said buildup inducing vibration in said firstand second bridges, the frequency of vibration being related to a strainimposed on the diaphragm.
 2. The pressure transducer as recited in claim1, wherein the improvement further comprises,means, disposed adjacent tothe bridges, for sensing the vibration frequency of the bridges bymeasuring the frequency of the variation in the intensity of lightenergy within said cavity.